U.S. patent number 5,147,903 [Application Number 07/681,815] was granted by the patent office on 1992-09-15 for dental materials.
This patent grant is currently assigned to Bayer Aktiengesellschaft. Invention is credited to Fritjof Bley, Wolfgang Podszun.
United States Patent |
5,147,903 |
Podszun , et al. |
* September 15, 1992 |
**Please see images for:
( Certificate of Correction ) ** |
Dental materials
Abstract
Dental materials contain a filler which consists of polymeric
crosslinked (meth)-acrylates having a particle size in the range
from 0.001 to 100 .mu.m, a degree of swelling of 50 to 2,000% by
weight and a degree of crosslinking of 1 to 100% by weight,
(meth)-acrylic acid esters which form crosslinkages and no
crosslinkages, and additives.
Inventors: |
Podszun; Wolfgang (Cologne,
DE), Bley; Fritjof (Achberg-Liebenweiler,
DE) |
Assignee: |
Bayer Aktiengesellschaft
(Leverkusen, DE)
|
[*] Notice: |
The portion of the term of this patent
subsequent to June 26, 2007 has been disclaimed. |
Family
ID: |
6356671 |
Appl.
No.: |
07/681,815 |
Filed: |
April 8, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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362722 |
Jun 7, 1989 |
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Foreign Application Priority Data
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Jun 16, 1988 [DE] |
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3820498 |
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Current U.S.
Class: |
523/115; 525/309;
523/116 |
Current CPC
Class: |
A61K
6/20 (20200101); A61K 6/891 (20200101); A61K
6/893 (20200101); A61K 6/887 (20200101); A61K
6/20 (20200101); C08L 75/16 (20130101); A61K
6/20 (20200101); C08L 63/10 (20130101); A61K
6/891 (20200101); C08L 63/10 (20130101); A61K
6/893 (20200101); C08L 75/16 (20130101); A61K
6/887 (20200101); C08L 33/00 (20130101); A61K
6/20 (20200101); C08L 33/00 (20130101); A61K
6/20 (20200101); C08L 75/16 (20130101); A61K
6/20 (20200101); C08L 63/10 (20130101); A61K
6/20 (20200101); C08L 33/00 (20130101); A61K
6/893 (20200101); C08L 75/16 (20130101); A61K
6/891 (20200101); C08L 63/10 (20130101); A61K
6/887 (20200101); C08L 33/00 (20130101) |
Current International
Class: |
A61K
6/02 (20060101); A61K 6/09 (20060101); A61K
6/083 (20060101); C08F 265/04 (); A61C
013/00 () |
Field of
Search: |
;524/307,308
;523/115,116 ;525/309 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bleutge; John C.
Assistant Examiner: Sweet; Mark
Attorney, Agent or Firm: Sprung Horn Kramer & Woods
Parent Case Text
This is a continuation of application Ser. No. 362,722, filed Jun.
7, 1989 now abandoned.
Claims
What is claimed is:
1. A composition suitable for forming a dental component,
consisting essentially of methacrylates and from 0.1 to 10 parts by
weight of one or more additives, the methacrylates consisting
essentially of about (a) 5 to 35 parts of a filler comprising a
polymeric crosslinked (meth)-acrtylate having a particle size of
about 0.01 to 10 .mu.m, a degree of swelling of about 100 to 2,000%
by weight and a degree of crosslinking of about 50 to 100% by
weight, in each case based on the polymer, and (b) 40 to 90 parts
of a (meth)-acrylate which can form crosslinkages.
2. A composition according to claim 1, wherein the filler has a gel
content of about 90 to 100% by weight.
3. A composition according to claim 1, wherein the filler has an
active surface area of about 20 to 600 m.sup.2 /g.
4. A composition according to claim 1, wherein (b) has a viscosity
of about 50 to 5,000 mPa.s.
5. A composition according to claim 4, wherein the filler has a gel
content of about 90 to 100% by weight.
Description
The invention relates to dental materials, their preparation and
their use.
Dental materials can be used, for example, for the production of
false teeth, crowns, bridges, inlays, onlays, dental fillings and
dental lacquers.
The preparation of dental materials based on polymeric
(meth)-acrylates is known. Thus, for example, materials which
contain polymethyl methacrylate bead polymers as the powder
component and mixtures of methyl methacrylate and ethylene
dimethacrylate are prepared; the mixtures in general harden by free
radical polymerization, with shaping (Ullmann's Encyclopedia of
Industrial Chemistry, Fifth Edition, Volume A8, p. 277 et seq. VCH
Verlagsgesellschaft m.b.H., Weinheim 1987).
Dental materials which, in addition to non-crosslinked polymethyl
methacrylate, contain a crosslinked polymethyl methacrylate in the
form of a bead polymer as a filler are described in U.S. Pat. No.
4,396,374 to Roemer et. al. These materials are built up by the
"interpenetrating plymer network" (IPN) principle. IPN systems are
known (J. Polym. Science 12, 141 (1977), J. Polym. Science 16, 583
(1978)).
The materials known from this reference do not have adequate
mechanical properties and cannot be processed in layers of any
desired thickness (for example in dental lacquers).
Dental materials containing a) 5 to 35 parts by weight of a filer
consisting of polymeric crosslinked (meth)-acrylates having a
particle size in the range from 0.001 to 10 .mu.m, a degree of
swelling of 50 to 2,000% by weight and a degree of crosslinking of
1 to 100% by weight, in each case based on the polymer, b) 40 to 90
parts by weight of (meth)-acrylates which can form crosslinkages,
c) 0 to 40 parts by weight of (meth)-acrylates which cannot form
crosslinkages and d) 0.1 to 10 parts by weight of one or more
additives, have been found.
Surprisingly, the dental materials according to the invention are
distinguished by a marked hardness and rigidity. They can be
processed to very thin layers.
(Meth)acrylates in the context of the present invention are esters
of acrylic acid and/or of methacrylic acid. Esters of methacrylic
acid are preferred.
Component (a)
Fillers in the context of the invention are polymeric crosslinked
(meth)-acrylates having a particle size in the range from 0.001 to
100 .mu.m, preferably 0,01-10 .mu.m, a degree of swelling of 50 to
2,000% by weight and a degree of crosslinking of 1 to 100% by
weight, in each case based on the polymer.
In the fillers, the poly(meth)acrylates according to the invention
have a degree of crosslinking of 1 to 100% by weight, preferably 50
to 100% by weight.
The degree of crosslinking is defined here as the percentage
(content) of methacrylic acid esters which can form crosslinkages
based on the polymer.
Monomeric (meth)-acrylates, in the fillers, which form
crosslinkages are (meth)-acrylates having 2 or more, preferably 2
to 4, polymerizable double bonds in the molecule.
Examples which may be mentioned of monomeric (meth)-acrylates which
form crosslinkages are: ethylene glycol dimethacrylate, diethylene
glycol dimethacrylate, triethylene glycol dimethacrylate, glycerol
dimethacrylate, glycerol trimethacrylate, trimethylolpropane
trimethacrylate, pentaerythritol dimethacrylate, pentaerythritol
trimethacrylate, pentaerythritol tetramethacrylate, derivatives of
bisphenol A, such as bisphenol A dimethacrylate and bisphenol A
diglycidyl dimethacrylate, urethane methacrylates which can be
prepared by reaction of diisocyanates and hydroxyalkyl
methacrylates, such as ##STR1## and reaction products of polyols,
dissocyanates and hydroxyalkyl methactrylates (DE-A 3,703,080, DE-A
3,703,130 and DE-A 3,703,120), such as, for example, ##STR2##
Monomeric (meth)-acrylates which form crosslinkages, such as
ethylene glycol dimethacrylate, diethylene glycol dimethacrylate,
triethlene glycol dimethacrlate and glycerol dimethacrylate, are
preferred.
Examples which may be mentioned of monomeric (meth)-acrylates, in
the fillers, which do not form crosslinkages are C.sub.1 -C.sub.12
-, preferably C.sub.1 -C.sub.4 -alkyl methacrylates, such as methyl
methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl
methacrylate, n-butyl methacrylate and t-butyl methacrylate,
hydroxalkyl (C.sub.1 -C.sub.4) methacrylates, such as
2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate,
diethylene glycol monomethacrylate and triethylene glycol
monomethacrylate, and alkoxy (C.sub.1 -C.sub.4)ethyl methacrylates,
such as 2-methoxyethyl methacrylate, 3-methoxybutyl methacrylate
and ethyltriglycol methacrylate.
Examples of preferred monomeric (meth)-acrylates which do not form
crosslinkages are methyl methacrylate, ethyl methacrylate and
2-hydroxyethyl methacrylate.
The monomeric (meth)-acrylates are known per se and can be
prepared, for example, by reaction of (meth)acrylyl chloride with
the corresponding alcohols.
It is of course possible for the (meth)-acrylates according to the
invention in the fillers to form copolymers with other monomers.
Examples which may be mentioned here are copolymers with styrene,
.alpha.-methylstyrene, acrylonitrile and vinyl acetate. In these
cases, the content of the comonomer is 0 to 40, preferably 0 to 20%
by weight, based on the polymer.
The degree of swelling is understood as the uptake capacity of the
poly(meth)acryaltes according to the invention for liquid. The
degree of swelling is measured by the uptake capacity for
tetrahydrofuran at 20.degree. C. The poly(meth)acryaltes according
to the invention have a swelling capacity of 50 to 2,000% by
weight, preferably 100 to 1,000% by weight, based on the
polymer.
The fillers according to the invention in general have an average
particle diameter of 0.001 to 100 .mu.m, preferably 0.01 to 10
.mu.m.
The fillers according to the invention preferably have a gel
content of 5 to 100% by weight, preferably 95 to 100% by weight,
based on the polymer. In the context of the present invention, the
gel content is understood, according to the invention, as the
content of the polymer which is insoluble in tetrahydrofuran as the
solvent at 20.degree. C. The gel content is a parameter of the
crosslinking which has actually occurred.
The fillers according to the invention preferably have an active
surface area of 20 to 600 m.sup.2 /g, preferably of 50 to 300
m.sup.2 /g, measured by the BET method.
Fillers according to the invention having a particle a size of
about 5-100 .mu.m can be synthesized by the suspension
polymerization process.
Preferred fillers according to the invention having a particle size
of 0.001 to 10 .mu.m can be prepared by polymerizing, as monomers,
(meth)-acrylates which form crosslinkages and if appropriate
(meth)-acrylates which do not form crosslinkages, and if
appropriate further commoners, in the presence of an organic
solvent with a solubility parameter of 8 to 15 [cal.sup.0.5
cm.sup.-1.5 ], the monomer content of (meth)-acrylates which form
crosslinkages being 50 to 100% by weight.
Organic solvents can be defined by the so-called solubility
parameter (H. G. Elias, Makromolekule, pages 192-196 (1981)).
Solvents having a parameter of 8 to 15 [cal.sup.0.5 cm.sup.-1.5 ],
preferably 8.5 to 12 [cal.sup.0.5 cm.sup.-1.5 ], are used for the
process according to the invention.
The following solvents may be mentioned as examples: amyl acetate,
tetrachloroethane, toluene, ethyl acetate, tetrahydrofuran,
benzene, chloroform, methylene chloride, methyl chloride, acetone,
butan-2-one and tert.-butanol.
The ratio of the amounts of solvent to monomeric (meth)-acrylates
is in the range from 1:1 to 1:100, preferably 1:2 to 1:20.
The polymerization process according to the invention is in general
carried out in the temperature range from 50.degree. to 250.degree.
C., preferably 60 to 150.degree. C. This polymerization can be
carried out continuously or discontinuously.
The polymerization is in general carried out in the presence of
initiators, such as sensitizers or agents which form free
radicals.
The initiators are in general employed in an amount of 0.01 to 3%
by weight, preferably 0.1 to 1.5% by weight, based on the total
monomer.
Polymerization initiators which can be used are, for example,
per-compounds, or azo compounds which supply free radicals. Example
which may be mentioned are aliphatic azodicarboxylic acid
derivatives, such as azobisisobutyronitrile or azodicarboxylic acid
esters, peroxides, such as lauroyl peroxide, succinyl peroxide,
dibenzoyl peroxide, oxide, p-chlorobenzoyl peroxide,
2,4-dichlorobenzoyl peroxide and peroxides such as methyl ethyl
ketone peroxide, methyl isobutyl ketone peroxide, cyclohexanone
peroxide and acetylacetone peroxide, alkyl esters of peracids, such
as tert.-butyl peripivalate, tert.-butyl peroctoate, tert.-butyl
perbenzoate, tert.-butyl perisononate, mono-tert.-butyl permaleate
and tert.-butyl peracetate, percarbonates, such as dicyclohexyl and
disopropyl percarbonate, dialkyl peroxides, such as di-tert.-butyl
peroxide and dicumyl peroxide, hydroperoxides, such as tert.-butyl
or cumene hydroperoxide, isophthalic mono-peracid or
acetylcyclohexanesulphonyl peroxide.
A suspension of the filler is in general formed in the
polymerization according to the invention. The filler can be
isolated, for example, by evaporation of the solvent, for example
in a spray drying process.
Component(b)
(Meth)-acrylic acid esters which can form crosslinkages in general
contain two or more polymerizable active groups, for example double
bonds or isocyanate groups, in the molecule. Esters of
(meth)-acrylic acid with 2- to 5-hydric alcohols having 2 to 30
carbon atoms may be mentioned as preferred. Epoxide methacrylates
and urethane methacrylates are particularly preferred.
(Meth)-acrylic acid esters of the formula ##STR3## in which A
denotes a straight-line chain, branched or cyclic, aliphatic,
aromatic or mixed aliphatic-aromatic radical having 2 to 25 C atoms
which can be interrupted by --0-- or NH bridges and substituted by
hydroxyl, oxy, carboxyl, amino or halogen,
R denotes H or methyl and
n stands for an integer from 2 to 8, preferably 2 to 4,
may be mentioned as examples.
Compounds of the following formulae may be mentioned as preferred:
##STR4## in the ortho, meta or para form ##STR5## wherein R stands
for ##STR6## n denotes a number from 1 to 4 and m denotes a number
from 0 to 5.
Derivatives of tricyclodecane (EP-A 0,023,686) and reaction
products of polyols, diisocyanates and hydroxyalkyl methacrylates
(DE-A 3,703,120, DE-A 3,703,080 and DE-A 3,703,130) may also be
mentioned. The following monomers may be mentioned as examples:
##STR7##
So-called bis-GMA of the formula ##STR8## is particularly preferred
as the monomer.
It is of course possible to use mixtures of various (meth)-acrylic
acid esters which can form crosslinkages. Mixtures of 20 to 70
parts by weight of bis-GMA- and 30 to 80 parts by weight of
triethylene glycol dimethacrylate may be mentioned as examples.
Component (c)
(Meth)-acrylic acid esters which cannot form crosslinkages are in
general monofunctional (meth)-acrylates. Examples which may be
mentioned are C.sub.1 -C.sub.12 -, preferably C.sub.1 -C.sub.4
-alkyl methacrylates, such as methyl methacrylate, ethyl
methacrylate, n-propyl methacrylate, i-propyl methacrylate, n-butyl
methacrylate and tert.-butyl methacrylate, hydroxyalkyl
methacrylates, such as 2-hydroxyethyl methacrylate, 2-hydroxypropyl
methacrylate, diethylene glycol monomethacrylate and triethylene
glycol monomethylmethacrylate, and alkoxyalkyl methacrylates, such
as 2-methoxyethyl methacrylate, 3-methoxybutyl methacrylate and
ethyl triglycol methacrylate.
Preferred (meth)-acrylic acid esters which cannot form
crosslinkages are methyl methacrylate, ethylene metharcylate and
hydroxyethyl methacrylate.
In general, a mixture of components (b) and (c) is employed as the
monomer mixture for the dental materials according to the
invention. These mixtures preferably have a viscosity in the range
from 50 to 5,000 mPa.s, preferably in the range from 100 to 2,000
mPa.s (in each case at 20.degree. C.).
Component (d)
The dental materials according to the invention can contain
additives which are known per se. Additives which may be mentioned
are starter additives, stabilizers, fillers, pigments, dyestuffs,
light stabilizers, fluorescent agents or plasticizers.
Starter additives which can be used for initiating the
polymerization are starter systems which are known per se
(literature Houben Weyl, Methoden der organischen Chemie (Methods
of Organic Chemistry) Volume E20, page 15 et seq., Georg Thieme
Verlay, Stuttgart 1987). These are systems which supply free
radicals, anions or cations and can initiate free radical, anionic
or cationic polymerization. In the case of systems which supply
free radicals, peroxides or aliphatic azo compounds, for example
benzoyl peroxide, lauryl peroxide or azoisobutyrodinitrile, are
particularly suitable; the systems are usually employed in amounts
of 0.1 to 5% by weight. While hardening at elevated temperature can
be carried out merely by means of peroxides or other free radical
starters, it is generally advantageous to add accelerators,
preferably aromatic amines, for hardening at room temperature.
Suitable accelerators are, for example, N,N-substituted toluidines
and xylidines, such as N,N-dimethyl-p-toluidine or
N,N-bis-(2-hydroxy-ethyl)-xylidine. Hardening can in general be
achieved by addition of 0.5 to 3% by weight of the amines
mentioned.
However, it is also possible to prepare dental materials which
polymerize under the action of light, for example UV light, visible
light or laser light. In these cases, photopolymerization
initiators and accelerators are employed.
Photopolymerization initiators are known per se (literature: Houben
Weyl, Methoden der organischen Chemie (Methods of Organic
Chemistry), Volume E20, page 80 et seq., Georg Thieme Verlag
Stuttgart 1987). They are preferably carbonyl compounds, such as
benzoin and derivatives thereof, in particular benzoin methyl
ether, benzyl and benzyl derivatives, for example 4,4-oxidibenzyl,
and other dicarbonyl compounds, such as thiacetyl, 2,3-pentadione
or metal carbonyls, such as pentacarbonyl-manganese, and quinones,
such as 9,10-phenanthrenequinone and camphorquinone or derivatives
thereof.
The content of such photopolymerization initiators is preferably
about 0.01 to about 5% by weight of the total composition.
The photopolymerizable compositions which can be hardened by means
of light preferably also contain substances which accelerate the
polymerization reaction in the presence of photopolmerization
initiators. Known accelerators are, for example, aromatic amines,
such as p-toluidine and dimethyl-p-toluidine, trialkylamines, such
as trihexylamine, polyamines, such as
N,N,N',N'-tetraalkylalkylenediamine, barbituric acid and
dialkylbarbituric acid, and sulphimides.
The accelerators are in general employed in an amount of 0.01 to
about 5% by weight of the total mixture.
It is also possible to add UV-stabilizers to the dental materials
according to the invention in order to avoid subsequent darkening
during ageing.
A particularly suitable UV stabilizer is
2-hydroxy-4-methoxybenzophenone. Another preferred material is
2-(2'-hydroxy-5-methylphenyl)-benzotriazole. Hydroquinone,
p-benzoquinone and p-butylhydroxytoluene may also be mentioned as
examples.
The dental materials according to the invention can also contain
pigments and dyestuffs which are known per se to establish a color
which is as true to nature possible.
Preferred dental materials according to the invention contain a) 5
to 35 parts by weight of a filler containing polymeric crosslinked
(meth)-acrylates having a particle size in the range from 0.001 to
100 .mu.m, a degree of swelling of 100 to 2,000% by weight and a
degree of crosslinking of 1 to 100% by weight, in each case based
on the polymer, b) 40 to 90 parts be weight of (meth)-acrylates
which can form crosslinkages, c) 0 to 40 parts by weight of
(meth)-acrylates which cannot form crosslinkages and d) 0.1 to 10
parts by weight of one or more additives.
Dental materials containing a) 10 to 30 parts by weight of a filler
containing polymeric crosslinked (meth)-acrylates having a particle
size in the range from 0.001 to 100 .mu.m, a degree of swelling of
100 to 2,000% by weight and a degree of crosslinking of 1 to 100%
by weight, in each case based on the polymer, b) 50 to 90 parts by
weight of (meth)-acrylates which can form crosslinkages, c) 0 to 40
parts by weight of (meth)-acrylates which cannot form crosslinkages
and d) 0.1 to 10 parts by weight of one or more additives, are
particularly preferred.
A process has also been found for the preparation of the dental
materials according to the invention, which is characterized in
that a mixture of a) 5 to 35 parts by weight of a filler consisting
of polymeric crosslinked (meth)-acrylates having a particle size in
the range from 0.001 to 10 .mu.m, a degree of swelling of 100 to
2,000% by weight and a degree of crosslinking of 1 to 100% by
weight, in each case based on the polymer, b) 40 to 90 parts by
weight of (meth)-acrylates which can form crosslinkages, c) 0 to 40
parts be weight of (meth)-acrylates which cannot form crosslinkages
and d) 0.1 to 10 parts by weight of additives, is polymerized.
The polymerization is in general carried out under the
abovementioned conditions.
The dental materials according to the invention can be processed by
shaping to give false teeth and dental prostheses, such as crowns,
bridges, inlays and onlays, and to dental fillings and dental
lacquers. The polymerized dental materials according to the
invention are distinguished by a favorable combination of
properties. They have a marked hardness, a high rigidity and a high
resistance to abrasion, coupled with good toughness. They can
easily be colored and can be adjusted to the natural tooth
color.
EXAMPLE 1
Preparation of a polymethacrylic acid ester of ethylene glycol
dimethacrylate
1,800 g of butan-2-one, 200 g of ethylene glycol dimethacrylate and
2 g of dibenzoyl peroxide are weighed into a 3 liter glass reactor
fitted with a blade stirrer, reflux condenser, internal
thermometer, gas inlet and gas outlet tube. The mixture is heated
under reflux for 2 hours, while stirring at 300 revolutions per
minute and flushing with nitrogen. An easily stirrable suspension
results. 190 g of fine powder can be obtained from this by spray
drying. The average particle size (measured by laser correlation
spectroscopy) is 700 nm, the gel content is 98.4% and the degree of
swelling (measured in tetrahydrofuran) is 310%.
EXAMPLE 2
Preparation of a polymethacrylic acid ester from glycerol
dimethacrylate
500 g of glycerol dimethacrylate and 5 g of dibenzoyl peroxide were
reacted in 2,000 g of butan-2-one in accordance with the procedure
described in Example 1. 475 g of powder having a particle size of
350 nm, a gel content of 97.3% and a degree of selling of 280% are
obtained.
EXAMPLE 3
Polymerizable composition
104 g of polymer from Example 1, 248 g of bis-GMA, 152 g of
triethylene glycol dimethacrylate and 2.1 g of dibenzoyl peroxide
are kneaded in a laboratory kneader in the course of 30 minutes.
The resulting composition is stored at 35.degree. C. for 5 hours. A
transparent, non-tacky, dough-like paste is obtained.
EXAMPLE 4
Polymerizable composition
36 g of polymer from Example 2, 120 g of the reaction product of
2,2,4-trimethylhexamethylene diisocyanate and 2 mol of
2-hydroxyethyl methacrtylate
[1,6-bis-(methacryloyloxyethoxycarbonylamino)-2,2,4-trimethylhexane]
and 0.64 g of dibenzoyl peroxide are kneaded to a composition as
described in Example 3.
EXAMPLE 5
20 g of polymer from Example 2, 60 g of bis-GMA, 30 g of
triethylene glycol dimethacrylate, 10 g of 4-methoxybutyl
methacrylate and 0.5 g of dibenzoyl peroxide are kneaded to a
composition as described in Example 3.
EXAMPLE 6
The compositions from Examples 3, 4 and 5 and two comparison
materials were polymerized at 140.degree. C. under 200 bar in the
course of 10 minutes to give a test sheet. Values of the flexural
strength according to DIN 13 922 and the flexural E modulus
according to DIN 13 922, and the penetration depth according to
Wallace were determined.
The Wallace method is used to determine the impression hardness of
plastics. A Vickers diamond is applied to the surface under a
preload of 1 p and is then subjected to a main load of 100 p for 60
seconds. The penetration depth of the diamond under the main load
is measured as a measure of the penetration resistance. In contrast
to the Vickers or Brinell hardness measurements, in which the test
force if related to the dimensions of the impression which remains,
the Wallace method records the elastic and permanent deformation of
the plastic.
______________________________________ Flexural Strength E modulus
HW N/nm.sup.2 N/nm.sup.2 .mu.m
______________________________________ Example 3 140 .+-. 7 3800
.+-. 150 16.2 .+-. 0.9 Example 4 136.1 .+-. 11 4100 .+-. 70 15.8
.+-. 0.5 Example 5 132 .+-. 10 3850 .+-. 130 17.0 .+-. 0.6
customary PMMA system 112 .+-. 9 2800 .+-. 100 22.8 .+-. 1.5 WO
82/02556 118 .+-. 10 3050 .+-. 80 20.3 .+-. 1.1 Example 1
______________________________________
It will be appreciated that the instant specification and claims
are set forth by way of illustration and not limitation, and that
various modifications and changes may be made without departing
from the spirit and scope of the present invention.
* * * * *